Thanks for the photos and for getting back to us.
Wire labels such as A, B, C, and D indicate a proprietary system.
Proprietary systems usually only work with thermostats made by the same manufacturer. But some proprietary systems may also have the option to use standard heating system wires. Contact a local pro and have them check if your proprietary system can be rewired to be Nest compatible.
I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.